In a variety of analytical instruments, depth profiling of the constituents of a material is done by surface analysis or analysis of a thin surface region in combination with an etching process that exposes subsequent depths of material to the analysis process. For example in secondary ion mass spectrometry (SIMS) analysis, depth profiling is typically done by using an ion beam to etch away successive surface thicknesses while (or by turns) analyzing the surface elements by probing with an ion beam to produce the emission of secondary ions from the surface. The secondary ions are analyzed by mass spectrometric techniques to identify them. In another example, X-ray photoelectron spectroscopy (XPS) sometimes also called electron spectroscopy for chemical analysis (ESCA), a similar etching sequence to permit depth profiling is combined with the use of an x-ray probe beam for analysis of the exposed surface. The x-ray probe beam stimulates release of photoelectrons from the surface and shallow subsurface. The kinetic energy spectra of the photoelectrons contain information from which elemental and chemical descriptions of the material can be estimated. For all of these methods, a confounding factor is the fact that the ion beam used for etching inevitably penetrates the surface being analyzed, disturbing the surface, and displacing atoms in a way that can cause them to be detected at a depth that is not representative of the original distribution. Conventional ion beams normally use atomic or molecular ions that produce a knock-on effect that can drive target atoms much deeper into the target. They can also produce an amorphous layer in which there is general mixing of the original atoms throughout a relatively thick region.
In recent years there has been much interest and activity toward using a GCIB as the etching beam because the nature of GCIB etching results in very little knock-on and formation of a very thin amorphous mixed layer to confound the resolution of the depth measurement.
Ions have long been favored for use in many processes because their electric charge facilitates their manipulation by electrostatic and magnetic fields. This introduces great flexibility in processing. However, in some applications, the charge that is inherent to any ion (including gas cluster ions in a GCIB) may produce undesirable effects in the processed surfaces. GCIB has a distinct advantage over conventional ion beams in that a gas cluster ion with a single or small multiple charge enables the transport and control of a much larger mass-flow (a cluster may consist of hundreds or thousands of molecules) compared to a conventional ion (a single atom, molecule, or molecular fragment.) Particularly in the case of insulating materials, surfaces processed using ions often suffer from charge-induced damage resulting from abrupt discharge of accumulated charges, or production of damaging electrical field-induced stress in the material (again resulting from accumulated charges.) In many such cases, GCIBs have an advantage due to their relatively low charge per mass, but in some instances may not eliminate the target-charging problem. Furthermore, moderate to high current intensity ion beams may suffer from a significant space charge-induced defocusing of the beam that tends to inhibit transporting a well-focused beam over long distances. Again, due to their lower charge per mass relative to conventional ion beams, GCIBs have an advantage, but they do not fully eliminate the space charge transport problem.
It is therefore an object of this invention to provide a method of etching a surface that produces a reduced mixing layer to facilitate improved depth resolution when employed in an analytical instrument.